Just joined having been looking for any advice on the boot lid rattle.
Just had my car serviced and asked them to sort the rattle out as it was driving me mad.
As usual they have replaced the bumpers and sprayed with silicon.
Having had the look round and pulled things about, I think everyone may possibly be barking up the wrong tree.
My main suspect is the gas struts that open the boot. Mine have play in them and when wobbled it sounds very similar to the sound in the car.
When the boot is closed I think they move as the car travels and the sound is transmitted direct to the bodywork at each fixing point.
This would explain why the silicon and bumpers haven't made a difference because I don't believe they are the problem.
Tomorrow I am going to pack the struts to see if this stops it. If it does I want new ones fitted under warranty.
Will let you know how I get on!